1. Field of the Invention
This invention relates to methods of manufacture of multiple-element piezoelectric transducers, for example, suitable for ultrasonic imaging applications in medicine and elsewhere.
2. Description of the Prior Art
Multiple-element piezoelectric transducers are known for use in several applications, in particular for nondestructive imaging of the interior of structures. For example, endoscopic imaging probes have been proposed for numerous medical applications. In many such imaging applications, it is desired to reduce the size of the individual piezoelectric elements as much as possible, to allow operation at higher frequencies and thereby provide increased resolution in the image.
Most present day piezoelectric transducers are fabricated of lead zirconate-titanate (PZT) ceramic material. The PZT material is commonly prepared from a PZT powder produced by mixing individual oxides (PbO, TiO.sub.2, ZrO.sub.2, plus small quantities of modifiers) and "calcining" the mixture. The calcination step is accomplished by heating the mixture to a high enough temperature (e.g., 800.degree.-900.degree. C.) to cause the constituent oxides to react to form single phase PZT in solid solution. The exact proportions of the constituent powders are varied in accordance with the specific properties desired in the PZT material being prepared. The calcined PZT is then ground to a fine powder, e.g., by conventional ball milling techniques. Dense blocks of PZT ceramic in desired shapes can then be formed by compacting the PZT powder in a die and then "sintering" the resultant "green" block at a suitable temperature (typically 1200.degree.-1300.degree. C.). Hot pressing, in which the PZT powder is subjected to external pressure during sintering, is also an effective way to produce blocks of PZT ceramic.
During the sintering process, some of the Pb content tends to be driven off, particularly from the surface of the pellet, with adverse effects on the properties of the ceramic; accordingly, it is known to place a quantity of Pb-containing material, such as PbZrO.sub.3, in the sintering furnace during the sintering step. Alternatively, the surface of the sintered block may be removed and discarded, after which the internal material is sliced into thin plates which are then lapped flat to form PZT plates for further processing, such as dicing into small individual elements for assembly into multiple-element arrays.
U.S. Pat. Nos. 4,514,247 and 4,572,981 to Zola disclose one proposed alternative in fabrication of multiple-element PZT transducer arrays. A single plate of PZT material prepared generally as above is diced into smaller slabs, which are then stacked interposed with layers of a passive material, such as glass, paper, phenolic resin, silicone rubber, or another ceramic material. The PZT slabs may be bonded to the layers of passive material using epoxy cement. The laminate block thus formed is then sliced perpendicular to the plane of the layers, resulting in plates comprising alternating strips of PZT and the passive material. These plates are then stacked, again interposed with layers of the passive material, and this assembly is sliced perpendicular to the strips, to yield one or more planar members in which PZT elements are surrounded by passive material. Single or separately-addressable electrodes are then applied to the opposed planar ends of the planar members.
Production of multiple-element transducer arrays according to the Zola method requires that the PZT plates be lapped to a desired thickness. As the lapping process requires a minimum strength, PZT elements manufactured according to the Zola process have an undesirably large minimum thickness. Zola's method also requires that individual members of the passive material be separately bonded to the PZT elements, which is a time-consuming and complex process. Zola also fails to teach an efficient method of connection of individual conductors to the individual PZT elements.
Other methods of fabrication of multiple-element arrays require multiple fine pitch dicing steps to form finely-divided PZT elements for subsequent assembly. Assembly of such small elements is very difficult. Moreover, the PZT material must be relatively fine grained (grain size .ltoreq.2 microns) to withstand the abrasive cutting technique employed in the dicing process. Relatively coarsely-grained PZT material (grain sizes .gtoreq.3-5 microns) exhibits higher piezoelectric sensitivity. Accordingly, At would be desirable to avoid dicing insofar as possible in the preparation of multiple-element PZT arrays, to allow use of the preferable coarsely-grained material.
A technique known as "tape-casting" is commonly employed for manufacture of "green" precursor tapes formed of materials which when sintered form a desired ceramic material. In tape-casting, a slurry is prepared including powders of the materials of the ceramic of interest, together with organic solvents and binders. A pool of the slurry, termed the "slip", is poured onto a moving substrate, between a stationary dam and one or two "doctor blades" extending parallel to the dam and spaced vertically from the moving substrate. As the substrate is pulled beneath the doctor blades, the thickness of the green precursor tape is precisely controlled by the spacing of the doctor blades from the substrate. Equivalently, tape casting can be performed by pulling one or a pair of mobile doctor blades across a stationary substrate. The green precursor is then dried and sintered to form the ceramic material. See, e.g., Runk and Andrejco, "A Precision Tape Casting Machine for Fabricating Thin Ceramic Tapes", Ceramic Bulletin, 54, No. 2, 199-200 (1975); Mistler, "Tape Casting: The Basic Process for Meeting the Needs of the Electronics Industry", Ceramic Bulletin, 69, No. 6, 1022-1026 (1990); Mistler, "Tape Casting", in Engineered Materials Handbook, 4, 161-165, (1992). See also U.S. Pat. No. 4,353,958 to Kita et al. Moreover, Wentworth and Taylor, in "Processing Parameters and Electric Properties of Doctor-Bladed Ferroelectric Ceramics", Ceramic Bulletin, 46, No. 12, 1186-1193, (1967), suggest that the properties of Pb(ZrSnTi)O.sub.3 ferroelectric ceramics prepared using doctor-bladed tape-casting techniques as above may be improved by disposing a source of Pb-containing material in the sintering furnace during the sintering step. More specifically, according to this technique, a green precursor tape is cut into strips and placed between Pb-rich "setter" members spaced about 2 mils from the surface of the green precursor strips. During sintering the presence of a Pb-containing setter increases the local Pb activity, thus minimizing Pb loss from the green tape and maintaining the proper ceramic composition.
Other references generally pertinent to the present invention include U.S. Pat. No. 4,939,826 to Shoup and U.S. Pat. No. 4,564,980 to Diepers, disclosing methods of manufacture of particular transducers; U.S. Pat. No. 4,977,655 to Martinelli, disclosing a method for mounting two transducers on a single catheter, for medical imaging purposes; U.S. Pat. No. 4,518,889 to 'T Hoen teaching an apodized transducer, that is, one having a particular asymmetric response; and U.S. Pat. No. 4,717,851 to Fenner et al, disclosing particular acoustic impedance matching layers for ultrasonic transducers.